Good afternoon and thank you for the opportunity to help in answering your question. Based on the information you have kindly provided I would go as far as to say that both your son and his fiance are addicted to Opiates. As for your first question, essentially what this means is that addicts will steal from whomever they can in order to get their fix. I realize that this doesn't make this easier to hear...it's not you that they are stealing from...it just happens that it's convenient given your relationship and proximity.
An addicts mindset is such that they when they need their next "fix" they will do whatever is necessary in order to achieve that high that they need. When I say need it's essentially a physiological need given that Opiates have a significantly difficult withdraw and it can be painful. Typically you see vomiting, hot and cold feelings (flu like), and severe muscular pain.
As for what you can do...I would suggest that you and her mother both confront your son and her daughter to let them know that you are both aware that they have been taking your medications and that you're concerned that they both have a problem and they would benefit from getting help. I would then suggest to you and her mother that you both hide your pills from now on some place they would never look for them...at least for the time being. If they continue to have access to the medications they will continue to steal from you.
I would also urge them to consider going to the hospital for treatment given the nature of the withdrawal. Unfortunately you cannot force them into treatment given their age and you may have to let them hit bottom on their own before recognizing that they do in fact need help.
I sincerely XXXXX XXXXX the answer I've provided you have been helpful and the suggestions I've offered have been of benefit to you.
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